The field of the present invention is colourless inorganic glasses which have a sharp optical absorption cutoff between 370 nm and 425 nm, as well as products manufactured from said glasses.
Such glasses, which are non-photochromic, and which are absorbers (of UV) up to a given wavelength beyond which, abruptly, they exhibit a maximum transmission (in the visible, around 400 nm), are known to the person skilled in the art.
They contain in their composition copper halides or copper cadmium halides. These elements, by precipitating in the form of micro-crystals, generally at the end of a thermal treatment of the glass prepared beforehand, are responsible for the particular properties of absorption of said glasses.
Such glasses have especially been described in the U.S. Pat. Nos. 5,281,562 and 5,322,819 patents. They can be presented in the form of  less than  less than colourless greater than  greater than  glasses or coloured glasses. Their coloration or  less than  less than non-coloration greater than  greater than , in the absence of specific colouring agents, does in fact depend upon the oxidation state of the copper within them; said oxidation state of the copper depending itself upon the melting temperature of the constituents of the glass, upon the partial oxygen pressure during said melting, upon the concentrations of polyvalent ions in the glass, upon the basicity of said glass, upon the reducing agent(s) content of said glass.
For the glass concerned, the following is generally observed
a green-blue coloration, when said copper is found mainly in the state of cupric ions : Cu++;
a red coloration, when said copper is found in the state of copper: Cu(o);
a  less than  less than non-coloration greater than  greater than , when said copper is found in the state of cuprous ions: Cu+.
It is obviously not excluded to incorporate colouring agents within said glasses, which colouring agents are intended to confer another coloration to them.
With reference to the preparation of the  less than  less than colourless greater than  greater than  glasses, there exists a real technical problem. The  less than  less than non-coloured greater than  greater than  glasses of the prior art always do in fact have a non-negligible residual yellow colour. This is strongly detrimental, especially in the context of specific uses of said glasses in which a strict transparency or a really colourless aspect is required.
Confronted with this technical problem of residual yellow coloration, the Applicant has developed novel inorganic glasses, of the type containing copper halides or copper cadmium halides, and having a sharp optical absorption cutoff between 370 nm and 425 nm, which are really  less than  less than colourless greater than  greater than 
In order to quantify this colourless character, i.e. the residual yellow coloration of an inorganic glass, the person skilled in the art is aware of the parameter: yellow index, which is notably measured according to the French Standard (NF): T51-067. Said French Standard is familiar to the person skilled in the art. According to this Standard, the yellow index is calculated by the formula:
(128 Xxe2x88x92106 Z)/Y,
with (X,Y,Z) which represent the colour points of said glass, colour points which are measured by using the illuminant C. Since this parameter depends upon the specific properties of absorption of the glass and obviously upon the thickness of the glass concerned, all the values compared in the present text, with reference to the glasses of the invention as well as with reference to the glasses of the prior art, are done so under comparable conditions, i.e. with glass samples of 2 mm thickness, said glasses having the same wavelength at 1% transmission (having the same absorption UV cutoff).
It is of course that the value of said absorption UV cutoff (wavelength at which 1% transmission is observed, below which the glass absorbs at more than 99%, and above which the glass hardly absorbs any more, given the sharp character of the optical absorption cutoff) depends both upon the composition of the glass and upon the thermal treatment that it has undergone. Classically, the glasses in question are in fact generally obtained in three steps:
a first step of formulation, in which their constituents are mixed;
a second step of melting and of pouring into a mould, at the end of which the glass is obtained in the adequate shape;
a third step of thermal post-treatment, during which the micro-crystals are prepared.
According to its first object, the present invention therefore relates to colourless inorganic glasses of the type containing copper halides or copper cadmium halides and having a sharp optical absorption cutoff between 370 nm and 425 nm. With reference to the qualifier  less than  less than colourless greater than  greater than , said glasses of the invention can be analysed as an improvement to the  less than  less than colourless greater than  greater than  glasses according to the U.S. Pat. Nos. 5,281,562 and 5,322,819 patents.
Said colourless glasses of the invention essentially have:
the composition below, expressed in cationic percentages:
the Cl, Br and F contents being themselves expressed in percentages by weight of the total composition;
with an adequate amount of reducing agent(s) which is (are) advantageously selected from SnO2, As2O3, Sb2O3, and mixtures thereof, in order to have mainly, in said glasses, the copper in the form of cuprous ions (Cu+), and therefore glasses which have neither a red coloration, nor a blue green coloration;
and furthermore, a minimised residual yellow coloration, i.e. measured according to Standard T51-067 (NF) recalled above, the yellow indices below:
a yellow index of less than 1.3, advantageously of less than 1, for a glass having a UV cutoff (wavelength at 1% transmission, measured on an optical transmission curve on a sample of said glass of 2 mm thickness) between 370 and 395 nm;
a yellow index of less than 2.3, advantageously of less than 2, for a glass having a UV cutoff (wavelength at 1% transmission, measured on an optical transmission curve on a sample of said glass of 2 mm thickness) at 400 nm;
a yellow index of less than 4, advantageously of less than 3, for a glass having a UV cutoff (wavelength at 1% transmission, measured on an optical transmission curve on a sample of said glass of 2 mm thickness) at 405 nm;
a yellow index of less than 20, advantageously of less than 12, and more advantageously still of less than 8 for a glass having a UV cutoff (wavelength at 1% transmission, measured on an optical transmission curve on a sample of said glass of 2 mm thickness) at 411 nm;
a yellow index of less than 50, advantageously of less than 20, and more advantageously still of less than 12, for a glass having a UV cutoff (wavelength at 1% transmission, measured on an optical transmission curve on a sample of said glass of 2 mm thickness) at 415 nm.